In reply to buxtoncoffeelover & Al Evans:
I think it gets referred to as the purest form of climbing because it's really just about the climbing and nothing else, you don't do it because you like exposure, or commitment, or as Al has said adventure, you do it for the enjoyment of climbing on rocks alone without any of those things.
Speaking of adventure Al, it might be all about that for you, but you could have an adventure doing a long list of things that aren't climbing, kayaking, caving, hiking to name a few. The only way you can rock climb is by climbing rocks and the way to do that which involves the least extraneous faff is bouldering.